tcp: tsq: relax tcp_small_queue_check() when rtx queue contains a single skb
commit f921a4a5bffa8a0005b190fb9421a7fc1fd716b6 upstream. In commit 75eefc6c59fd ("tcp: tsq: add a shortcut in tcp_small_queue_check()") we allowed to send an skb regardless of TSQ limits being hit if rtx queue was empty or had a single skb, in order to better fill the pipe when/if TX completions were slow. Then later, commit 75c119afe14f ("tcp: implement rb-tree based retransmit queue") accidentally removed the special case for one skb in rtx queue. Stefan Wahren reported a regression in single TCP flow throughput using a 100Mbit fec link, starting from commit 65466904b015 ("tcp: adjust TSO packet sizes based on min_rtt"). This last commit only made the regression more visible, because it locked the TCP flow on a particular behavior where TSQ prevented two skbs being pushed downstream, adding silences on the wire between each TSO packet. Many thanks to Stefan for his invaluable help ! Fixes: 75c119afe14f ("tcp: implement rb-tree based retransmit queue") Link: https://lore.kernel.org/netdev/7f31ddc8-9971-495e-a1f6-819df542e0af@gmx.net/ Reported-by: Stefan Wahren <wahrenst@gmx.net> Tested-by: Stefan Wahren <wahrenst@gmx.net> Signed-off-by: Eric Dumazet <edumazet@google.com> Acked-by: Neal Cardwell <ncardwell@google.com> Link: https://lore.kernel.org/r/20231017124526.4060202-1-edumazet@google.com Signed-off-by: Jakub Kicinski <kuba@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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1 changed files with 14 additions and 2 deletions
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@ -2497,6 +2497,18 @@ static bool tcp_pacing_check(struct sock *sk)
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return true;
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}
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static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
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{
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const struct rb_node *node = sk->tcp_rtx_queue.rb_node;
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/* No skb in the rtx queue. */
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if (!node)
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return true;
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/* Only one skb in rtx queue. */
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return !node->rb_left && !node->rb_right;
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}
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/* TCP Small Queues :
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* Control number of packets in qdisc/devices to two packets / or ~1 ms.
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* (These limits are doubled for retransmits)
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@ -2534,12 +2546,12 @@ static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
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limit += extra_bytes;
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}
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if (refcount_read(&sk->sk_wmem_alloc) > limit) {
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/* Always send skb if rtx queue is empty.
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/* Always send skb if rtx queue is empty or has one skb.
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* No need to wait for TX completion to call us back,
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* after softirq/tasklet schedule.
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* This helps when TX completions are delayed too much.
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*/
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if (tcp_rtx_queue_empty(sk))
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if (tcp_rtx_queue_empty_or_single_skb(sk))
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return false;
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set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
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